The invention relates to an ultrasonic measuring apparatus with transmitter and receiver for locating the geometrical position of the boundary between a first and a second material by measuring and comparing the propagation times required for return to the receiver by an ultrasonic signal emitted by the transmitter and reflected at the boundary to the first material and an ultrasonic signal reflected at the boundary to the second material.
Ultrasonic measuring apparatuses of the kind specified are known (DE 195 26 014 A1). Reliable results concerning the distance of boundary surfaces are obtained from the speed of propagation of an ultrasonic signal in the first material. Such ultrasonic measuring apparatuses are used for measuring wall thicknesses. Measurement is problem-free if the ultrasonic signal reflected at the boundary to the first material has already decayed when the signal reflected at the boundary to the second material reaches the receiver again. This precondition is met in the case of materials in which the boundary to the first material is far enough away from the boundary to the second material. In these cases the individual reflected signals can be selected in dependence on the sequence of their impingement and assigned to the boundary layers. This prior art does not solve the problem that the signal received is a mixed signal which, due to the small thickness of the individual layers, is composed of the signals reflected at the different layers. Such a signal cannot be readily analysed for layer thickness measurement.
For measuring the thickness of multilayer plastics pipes, etc., a process is also known (DE 195 34 503 A1) which is based not on the measurement of propagation time, but on the measurement of the amplitudes of the ultrasonic signal at resonance frequency.
Lastly, an ultrasonic measuring process is known (DE 34 18 486 C1) which is used not for measuring the thickness of thin layers/walls, but for eliminating faults close to the rear wall. This is done by the reflected signal being compared with the signal sample which is characteristic of the course of the rear wall echo up to its maximum extreme value of amplitude.
It is an object of the invention to provide an ultrasonic measuring apparatus also suitable for locating the geometrical position of a boundary between two materials which is situated very close to a place of reference, for example, another boundary between two materials, so that the measuring apparatus is also in a position to determine the thickness of very thin-walled materials.